Update the example for tsc target es2020

[dilyanpalauzov]

I put in package.json : "type":"module" and change in tsconfig: "target":"es2020". Then I run tsc && node src/index.js. Node says, that it cannot find src/counter/counter.

I change in src/index.ts the first line to import {Counter} from './counter/counter.js', the second line to import {someHelpfulFunction} from './util.js' and repeat tsc && node src/index.js. This works.

I delete src/*.jsand src/counter/counter.js and call rollup. It says Error: Could not resolve './util.js' from src/index.ts

I cannot write import {Counter} from './conter/counter.ts', because tsc does not allow this, see also microsoft/TypeScript#38546.

Please update the example, so that it works with TypeScript when target:es2020 and type:module are used and so that the same input files are understood the same way by the typescript compiler, by @rollup/plugin-babel/@babel/preset-typescript and by NodeJS.